In the field of wireless communications, a frequency spectrum may be a valuable property right. Frequency spectrums are often licensed to cellular telephone service providers, allowing them to provide wireless service to millions of customers using their equipment network. To maximize revenue from data traffic operating within their licensed frequency spectrum, cellular telephone service providers must densely pack user channels into their licensed frequency spectrum. Potential interference with adjacent channel traffic severely constrains the amount of radio-frequency energy which may be transmitted outside any assigned spectral channel. Cellular telephone equipment often cannot be approved for sale by government regulators unless its transmissions satisfy strict constraints on out-of-band transmitted energy.
Wireless transmitters must use power amplifiers to increase the power of the transmitted signal so that sufficient power is received at the other end of the data link. Power amplifier operation reflects a compromise between the efficiency with which battery power is converted into useful signal power and the degree of to which transmitted energy appears outside the assigned spectral channel. Power amplifiers often operate relatively inefficiently in order to satisfy regulatory constraints on out-of-band energy.
Two related art technologies in transmission technology attempt to address the tradeoff between efficiency and out-of-band energy. In one related art technology, envelope Tracking (ET) dynamically varies the level of the amplifier's power supply voltage, so it is always at the minimum level demanded for linear amplifier operation. However, this dynamic supply voltage causes the amplifier output impedance to also dynamically vary, inducing undesirable mismatch between the amplifier and subsequent transmitter components. A second related art technology dynamically varies output impedance of a device between a power amplifier and an antenna, attempting to minimize mismatches caused by antenna input impedance variations induced by objects moving in close proximity to the cellphone antenna. Any variation in impedance may result in a dynamic mismatch between the output impedance of one component and the input impedance of the next component. Mismatches may cause energy to be reflected and energy reflected back into the nonlinear amplifier increases out-of-band energy levels. The need exists for a method and/or apparatus to minimize out-of-band energy caused by dynamic impedance mismatches associated with either ET or techniques compensating for antenna input impedance variations due to proximate objects.